1. Field of the Invention
The present invention concerns a method to acquire and display calibration images in a periodically moving organ with the use of magnetic resonance technology. Furthermore, the invention concerns a magnetic resonance apparatus to implement such a method
2. Description of the Prior Art
Calibration images are used (among other things) in magnetic resonance imaging before diagnostically significant images of a subject to be examined are acquired and serve for the adjustment or, respectively, the optimization of the subsequent acquisition of the measurement data from which the diagnostically significant images are generated.
Magnetic resonance technology (in the following the term “magnetic resonance” is also shortened to MR) is thereby a technique that has been known for several decades with which images of the inside of an examination subject can be generated. Described in a significantly simplified way, the examination subject is positioned in a relatively strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla or more) so that nuclear spins in the subject orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to excite nuclear magnetic resonances, the resonant nuclear spin signal is measured, and MR images are reconstructed based thereon. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. By means of a multidimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated with such values.
MR signal generation and acquisition are sensitive to small errors and inaccuracies in the technique that is used. Depending on the measurement sequence that is used, artifacts in particular increasingly occur at high field strengths of 3 Tesla or more, which are increasingly being used for diagnostic imaging.
Effects known as the off-resonance effects are one cause of image artifacts. These effects occur when the resonance frequency (Larmor frequency) of the nuclear spins to be excited differs slightly from the frequency with which the excitation pulses are radiated. For example, this slight difference can be the consequence of an insufficient shim or an insufficient frequency adjustment. In the reconstructed image, this can often appear as band-shaped artifacts, which can considerably hinder an evaluation of measured data. The TrueFISP sequence (True fast imaging with steady state precession) is one example of a known and established sequence that is sensitive to off-resonance effects.
The use of calibration images to select a suitable offset frequency is presently known in order to combat this problem. The offset frequency specifies how significantly the actual excitation frequency of an excitation pulse deviates from a previously selected excitation frequency. Such calibration images are also known as “frequency scouts”. One image per heartbeat is acquired with a TrueFISP sequence over a breath-hold phase. The value for the offset frequency is varied in equal steps in a suitable frequency range across the heartbeats. The image with the best image quality can then be selected from the image series that is acquired in this way. The offset frequency associated with this image can then be used in the following TrueFISP measurement in which the diagnostic image data are then acquired with an advantageous artifact response.
FIG. 2 schematically shows the chronological workflow of this acquisition. Trigger points 47 that respectively identify the beginning of a cardiac cycle are determined between the beginning 41 and end 43 of a breath-hold phase based on the EKG signal 45. A calibration image 51 . . . 54 is respectively acquired at a defined time interval at these trigger points 47, wherein the calibration images differ in their offset frequency.